Optical systems have been extensively studied for object recognition. With rapid advances in CCD array sensors, computers and software, digital holography can be performed efficiently as an optical system for 3-D object recognition. Digital holography has also been a subject of great interest in various fields.
In digital holography, the off-axis scheme has been widely used since it is simple and requires only a single exposure in separating the desired real image term from the undesired DC and conjugate terms.
However, 3-D object recognition by use of the off-axis scheme has inherent limitations in terms of robustness to the variation of the 3-D object position due to the superposition of the real image with undesired terms.
The tolerance problem to the variation of the 3-D object positioning can be resolved by employing an on-axis scheme. For example, 3-D object recognition by use of phase shifting digital holography has provided a feasible approach for implementing on-axis 3-D object recognition.
The phase shifting digital holographic method requires multiple hologram recordings. Therefore, the 3-D object recognition system based on the on-axis phase shifting approach has inherent constraints in the sense that it needs a vibration-free environment that is convenient only in a laboratory. Also, it is sensitive to recording a moving object.